1. Field of the Invention
The invention relates to thermoplastic molding compositions based on a polyphenylene ether, a comblike polymer which serves to improve impact strength, and possibly additional polymers and additives.
2. Discussion of the Background
Polyphenylene ethers are thermoplastics which may be produced on a substantial industrial scale and have high viscosity and high softening temperature. They are suited for numerous industrial applications in which high heat deflection temperature (HDT) is desirable. These polycondensates are described in (among other publications) U.S. Pat. Nos. 3,306,874, 3,306,875, 3,257,357 and 3,257,358. However, polyphenylene ethers have the disadvantage of being brittle. In addition, they are not readily processible since due to their high softening temperatures and high melt viscosities one must employ high temperatures and high shear forces which can lead to their being damaged.
Accordingly, attempts have been made to improve the processibility of polyphenylene ethers and the impact strength of the products by admixing other polymers. Mixtures of polyphenylene ethers with high impact polystyrene (HIPS), such as described in, e.g., German No. AS 21 19 301, are industrially important because not only the processibility of the polyphenylene ether melt is improved but the impact strength of the molded parts produced from the compositions is increased. However, with this known approach good processibility is achieved at the expense of a substantial loss of HDT. Also, it has been found that the resulting molded pieces often do not have sufficient impact strength and sufficiently high HDT.
It has been proposed to increase impact strength by the addition of elastomeric block mixed polymers. Thus, e.g., in German No. AS 19 32 234 and in German Nos. OSS 24 34 848, 27 13 509 and 27 50 515, there are described molding compositions comprised of polystyrenes, polyphenylene ethers, and additives comprising variously structured hydrogenated and/or non-hydrogenated block mixed polymers. The block mixed polymers employed are comprised chiefly of polymerised vinylaromatic hydrocarbons and polymerized conjugated dienes.
In German No. OS 32 26 427, molding compositions comprised of polystyrenes, polyphenylene ethers, and additives comprising a block mixed polymer are described. The blocks of the block mixed polymer comprise polyethylene, polyacrylic acid esters or polymethacrylic acid esters, polymerized vinylaromatic hydrocarbons, or polymerized conjugated dienes.
In all these mixtures, the addition of the elastomeric block mixed polymer and polyolefin resins results in a decrease in the HDT, and an inadequate increase of the impact strength. Furthermore, the elasticizing components described are applicable only for mixtures which contain both polyphenylene ethers and polystyrene resins.
Products containing such mixtures of polyphenylene ethers and polystyrene resins inherently have low HDT. Thus, commercially available mixtures of polyphenylene ethers with HIPS generally have a HDT under heating to at maximum 150.degree. C.
There is thus a strongly felt need for a thermoplastic molding composition, based on polyphenylene ether, which can be readily processed into molded pieces having a high impact strength and having a good HDT.